This application is a continuation of international patent application no. PCT/EP00/04754, filed May 25, 2000, the entire disclosure of which is incorporated herein by reference. Priority is claimed based on Federal Republic of Germany patent application no. DE 199 27 135.6, filed Jun. 15, 1999.
The invention relates to a device for absorbing the axial thrust of a multistage centrifugal pump, with a balancing device which has one or more axial gaps and one or more radial gaps and in which a balancing stream is conducted through the gaps, and with an axial bearing absorbing the remaining residual thrust, the axial bearing having an associated cardanic ring which serves, among other things, to compensate errors of alignment.
In order to absorb the axial thrust in multistage centrifugal pumps, essentially three different types of balancing devices are known: balancing disk, balancing piston and stepped piston. The latter is constructed, in the form predominantly in use, as a double piston. These devices are described under the keyword xe2x80x9caxial thrustxe2x80x9d in the KSB Centrifugal Pump Lexicon, third edition, 1989.
What is common to all three versions is a balancing stream conducted via gaps. The balancing stream, which usually is conducted back to the inlet of the centrifugal pump, represents a leakage loss for the pump. Attempts have been made to minimize this leakage loss by using gap widths which are as small as possible. In such a case, however, care must be taken to assure that rubbing of the moving parts of the centrifugal pump against stationary parts of the pump is avoided as far as possible under all operating conditions. Rubbing of the pump rotor in the casing may lead to frictional wear on surfaces located opposite one another and ultimately to failure of the centrifugal pump.
The gap losses occurring in each case due to the balancing stream differ in amount in each of the three types of balancing device. Thus, the radial gap surrounding the single balancing piston results particularly in a large gap stream and therefore in a sharp decrease in efficiency of the centrifugal pump. By contrast, the gap loss of a stepped-piston balancing device, which, depending on the number of steps, has at least two radial gaps and in each case an axial gap arranged between the radial gaps, is substantially lower. What is common to both versions operating with pistons is that they also require an axial bearing which absorbs a remaining residual thrust.
The balancing device which comprises a balancing disk does not, in principle, require such an axial bearing, since the balancing disk has a self-regulating effect. Since the substantially axial gap between the balancing disk and the nonrotating counterdisk is set very narrow, the leakage loss of this type of device also remains relatively low. However, when centrifugal pumps with high output pressures and large capacities are operated in a transient mode of operation, the balancing disk may rub against the counterdisk. Thus, the operating reliability of such pumps is no longer assured.
It is an object of the invention to provide an improved balancing device for a multistage centrifugal pump.
Another object of the invention is to provide a balancing device for a multistage centrifugal pump which incurrs only minimal leakage losses and yet exhibits a high operating reliability.
These and other objects are achieved in accordance with the present invention by providing a balancing arrangment for absorbing axial thrust of a multistage centrifugal pump with a suction side and a pressure side, comprising a balancing device having at least one axial gap and at least one radial gap delimited by opposed faces of the balancing device and in which a balancing stream is conducted through the gaps, an axial bearing for absorbing remaining residual thrust, a cardanic ring associated with the axial bearing and serving to compensate for alignment errors, wherein said balancing device is constructed such that a residual thrust in the direction of the suction side of the pump exists in all operating states of the pump, and the cardanic ring is dimensioned such that it is elastically deformed by the residual thrust and has a spring constant such that, starting from a maximum gap width (s) when the pump is at rest, the axial gap closes under operating conditions to a minimum width at which contact between the opposed faces delimiting the axial gap is still avoided.
The balancing device of the present invention is constructed in such a way that, in all the operating states, a residual thrust occurs which acts in the direction of the suction side of the centrifugal pump, and such that the cardanic ring is dimensioned so that it is deformed elastically by the residual thrust. The spring constant of the cardanic ring is selected such that, starting from a maximum gap width when the centrifugal pump is in a state of rest, the axial gap closes under operating conditions to a minimum width at which contact between the faces delimiting the axial gap is still avoided. If the balancing device comprises a stepped piston with a plurality of axial gaps, this described condition applies to all the axial gaps.
However, the invention is not only useful with balancing devices comprising double pistons or multistep pistons. It may also be employed in the case of balancing disks. Admittedly, an additional axial bearing with a cardanic ring entails an increased expense in the case of a device with a balancing disk. However, since the operating reliability in centrifugal pumps with high output pressures and large capacities can thereby be assured, even in the case of a transient mode of operation, the expense is justified at least in this case.
The invention can be used especially advantageously conjunction with a hydrodynamic axial bearing.